Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, e.g., watch and calculator displays, as well as the flat-panel displays found in portable computers and compact televisions. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available.
One of the most important characteristics of a liquid crystal display device is its response time, i.e., the time required for the device to switch from the on (light) state to the off (dark) state. In a ferroelectric or anti-ferroelectric device, response time (.tau.=.eta./P.sub.s E) is proportional to the rotational viscosity (.eta.) of the liquid crystal compound(s) contained within the device and is inversely proportional to their polarization (P.sub.s) and to the applied electric field (E). Thus, response time can be reduced by using compound(s) having high polarizations or low viscosities, and such compounds are greatly desired in the art.
In the passive addressing of liquid crystal compounds exhibiting a spontaneous polarization, however, low polarization mixtures can be important for the practical operation of a liquid crystal device. Polarization reversal fields are larger for higher polarization mixtures, and polarization reversal fields cause switching or partial switching back to a material's original director alignment. This results in loss of the bistability that is crucial to the passive-matrix driving of ferroelectric liquid crystal devices.
Another potential disadvantage of using high polarization mixtures is the partial switching of their director alignment in response to non-switching (secondary) signals in a driving waveform. This continued response or fluctuation of the director causes a large decrease in the contrast ratio of a ferroelectric liquid crystal device.
In addition to fast response times, compounds should ideally possess broad smectic temperature ranges to enable operation of the device over a broad range of temperatures (or should be capable of combination with other liquid crystal compounds having different smectic temperature ranges without adversely affecting the smectic phase behavior of the base mixture).